Mileage Car

Measure

How to calculate and track your mileage

Your dashboard estimate is handy, but measuring at the pump tells you what the car really uses. It takes a few seconds per fill-up and, over a few weeks, shows exactly what raises and lowers your mileage.

The fill-up method

  1. Fill the tank and let the pump click off on its own. Don't top it up after the click, because that makes each fill less consistent.
  2. Note the odometer reading, or reset a trip meter to zero.
  3. Drive normally until you need fuel again.
  4. Fill up the same way, again stopping at the first click. Note the gallons from the pump and the new odometer reading.
  5. Divide the miles driven by the gallons added. That's your MPG for that tank.

One tank can be thrown off by a pump that clicks early or a slope at the station. Averages over several tanks are more reliable, and the overall average is simply total miles divided by total gallons.

Mileage calculator

Enter the distance and the fuel used since your last fill-up.

The calculator runs in your browser. Nothing you type is sent anywhere.

An example log

Here is the same example log from the home page with the metric figures added. The notes column is the most useful part. Writing one line about each tank, such as "highway trip" or "cold week", lets you match the swings to their causes.

Example log

Starting odometer 024180

30.2 MPG overall

FillOdometerMilesGallonsMPGL/100 kmAgainst averageWhat was going on
102449231210.430.07.8Mixed commute
202479029810.628.18.4Cold week, lots of short trips
302519540511.236.26.5Highway trip at about 65 mph
402556637112.928.88.2Highway at about 75 mph, roof box on
502587130510.130.27.8Mixed commute, tires set to placard
602613326210.325.49.2City errands, AC on high
702646333010.531.47.5Mixed commute, gentler starts
802678131810.231.27.5Mixed commute
MPG for each fill-up in the example log, with the overall average of 30.2 MPG marked2226303438average 30.2Fill 1: 30.0 MPG, Mixed commute1Fill 2: 28.1 MPG, Cold week, lots of short trips2Fill 3: 36.2 MPG, Highway trip at about 65 mph3Fill 4: 28.8 MPG, Highway at about 75 mph, roof box on4Fill 5: 30.2 MPG, Mixed commute, tires set to placard5Fill 6: 25.4 MPG, City errands, AC on high6Fill 7: 31.4 MPG, Mixed commute, gentler starts7Fill 8: 31.2 MPG, Mixed commute8
Made-up numbers to show the method. Green dots beat the running average; amber dots fall below it. The notes explain most of the swings.

Converting units

FromToHow
MPG (US)L/100 kmDivide 235.2 by the MPG
L/100 kmMPG (US)Divide 235.2 by the L/100 km figure
MPG (US)km/LMultiply by 0.425
MPG (US)MPG (UK)Multiply by 1.2, since a UK gallon is larger

Dashboard readouts

Most newer cars show average and instant MPG. These are estimates from the engine computer, and they're often a few percent optimistic. They're still useful for spotting trends and seeing how a change in driving shows up. Check them against your fill-up log from time to time.

Keeping mileage records for other reasons

People also track miles for work expenses, taxes or reimbursement. Rules and rates for those uses change and differ by country and employer. In the US, the IRS publishes current standard mileage rates and what records it expects, which generally means a record made at the time showing the date, the miles, where you went and why. Check the IRS website or ask a tax professional before you rely on a log for tax purposes.

  • Record the odometer at the start and end of each trip, or of each period.
  • Write the purpose and destination as you go, not from memory later.
  • Keep fuel receipts with the log if your employer or tax rules ask for them.

Tracking FAQ

Why does my MPG vary so much between tanks?

Weather, trip length, traffic and how full the pump fills the tank all move single-tank results. Look at the average across four or more tanks.

Should I use an app?

Any method works if you use it consistently. A paper notebook in the glove box is as accurate as an app. What matters is recording every fill-up.